This invention relates to a multicarrier CDMA transmission system and transmission method. More particularly, the invention relates to a multicarrier CDMA transmission system and transmission method for subjecting transmit data to a serial-to-parallel conversion, multiplying each symbol of the obtained parallel data individually by each code constituting orthogonal codes, and multicarrier-transmitting each result of multiplication by a prescribed subcarrier.
Multicarrier modulation schemes have become the focus of attention as next-generation mobile communication schemes. Using multicarrier modulation not only makes it possible to implement wideband, high-speed data transmission but also enables the effects of frequency-selective fading to be mitigated by narrowing the band of each subcarrier. Further, using orthogonal frequency division multiplexing not only makes it possible to raise the efficiency of frequency utilization but also enables the effects of inter-symbol interference to be eliminated by providing a guard interval for every OFDM symbol.
(a) in FIG. 10 is a diagram useful in describing a multicarrier transmission scheme. A serial/parallel converter 1 converts serial data to parallel data and inputs the parallel data to orthogonal modulators 3a to 3d via D/A converter 2a to 2d, respectively. In the Figure, the conversion is to parallel data comprising four symbols. Each symbol includes an in-phase component and a quadrature component. The orthogonal modulators 3a to 3d subject each of the symbols to orthogonal modulation by subcarriers having frequencies f1 to f4 illustrated in (b) of FIG. 10, a combiner 4 combines the orthogonally modulated signals and a transmitter (not shown) up-converts the combined signal to a high-frequency signal and then transmits the high-frequency signal. With the multicarrier transmission scheme, the frequencies are arranged, as shown at (b), in such a manner that the spectrums will not overlap in order to satisfy the orthogonality of the subcarriers.
In orthogonal frequency division multiplexing, frequency spacing is arranged so as to null the correlation between a modulation band signal transmitted by an nth subcarrier of a multicarrier transmission and a modulation band signal transmitted by an (n+1)th subcarrier. If we assume that a symbol (a complex baseband signal) transmitted by the nth subcarrier (center frequency: fn) is represented by zn (=an+jbn), then we may write modulation band signal sn(t)=Re[zn exp(j2πfn t)] (where Re represents the real part of the complex number). The requirement for the (n+1)th subcarrier to be orthogonal to the nth subcarrier is that the cross correlation between sn(t) and sn+1(t) be 0. If the frequency spacing between neighboring subcarriers is fd and the period of the symbol zn is T, then, in order for the cross correlation to become 0, it will suffice for fd=k/T (k=1, 2, . . . ) to hold and the minimum spacing will be fd=1/T. A multicarrier multiplexing scheme having frequency spacing is an orthogonal frequency division multiplexing scheme.
(a) of FIG. 11 is a diagram of the structure of a transmitting apparatus that relies upon the orthogonal frequency division multiplexing scheme. A serial/parallel converter 5 converts serial data to parallel data comprising a plurality of symbols (I+jQ, which is a complex number). An IDFT (Inverse Discrete Fourier Transform) 6, which is for the purpose of transmitting the symbols as subcarriers having a frequency spacing shown in (b) of FIG. 11, applies an inverse discrete Fourier transform to the frequency data to effect a conversion to time data, and inputs the real and imaginary parts to an orthogonal modulator 8 through D/A converter 7a, 7b. The orthogonal modulator 8 subjects the input data to orthogonal modulation, and a transmitter (not shown) up-converts the modulated signal to a high-frequency signal. In accordance with orthogonal frequency division multiplexing, a frequency placement of the kind shown in (b) of FIG. 11 becomes possible, thereby enabling an improvement in the efficiency with which frequency is utilized.
In recent years, there has been extensive research in multicarrier CDMA schemes (MC-CDMA) and application thereof to next-generation wideband mobile communications is being studied. With MC-CDMA, partitioning into a plurality of subcarriers is achieved by serial-to-parallel conversion of transmit data and spreading of orthogonal codes in the frequency domain. Owing to frequency-selective fading, subcarriers distanced by their frequency spacing experience independent fading on an individual basis. Accordingly, by causing code-spread subcarrier signals to be distributed along the frequency axis by frequency interleaving, a despread signal can acquire frequency-diversity gain.
An orthogonal frequency/code division multiple access (OFDM/CDMA) scheme, which is a combination of OFDM and MC-CDMA, also is being studied. This is a scheme in which a signal, which has been divided into subcarriers by MC-CDMA, is subjected to orthogonal frequency multiplexing to raise the efficiency of frequency utilization.
A CDMA (Code Division Multiple Access) scheme multiplies transmit data having a bit cycle Ts by spreading codes C1 to CN of chip frequency Tc using a multiplier 9, as shown in FIG. 12, modulates the result of multiplication and transmits the modulated signal. Owing to such multiplication, a 2/Ts narrow-band signal NM can be spread-spectrum modulated to a 2/Tc wideband signal DS and transmitted, as shown in FIG. 13. Here Ts/Tc is the spreading ratio and, in the illustrated example, is the code length N of the spreading code. In accordance with this CDMA transmission scheme, an advantage acquired is that an interference signal can be reduced to 1/N.
According to the principles of multicarrier CDMA, N-number of items of copy data are created from a single item of transmit data D, as shown in FIG. 14, the items of copy data are multiplied individually by respective ones of codes C1 to CN, which are spreading codes (orthogonal codes), using multipliers 91 to 9N, respectively, and products DC1 to DCN undergo multicarrier transmission by N-number of subcarriers of frequencies f1 to fN illustrated in (a) of FIG. 15. The foregoing relates to a case where a single item of symbol data undergoes multicarrier transmission. In actuality, however, as will be described later, transmit data is converted to parallel data of M symbols, the M-number of symbols are subjected to the processing shown in FIG. 14, and all results of M×N multiplications undergo multicarrier transmission using M×N subcarriers of frequencies f1 to fNM. Further, orthogonal frequency/code division multiple access can be achieved by using subcarriers having the frequency placement shown in (b) of FIG. 15.
FIG. 16 is a diagram illustrating the structure on the transmitting side of MC-CDMA. A data modulator 11 modulates transmit data and converts it to a complex baseband signal (symbol) having an in-phase component and a quadrature component. A time multiplexer 12 time-multiplexes the pilot of the complex symbol to the front of the transmit data. A serial/parallel converter 13 converts the input data to parallel data of M symbols, and each symbol is input to a spreader 14 upon being branched into N paths. The spreader 14 has M-number of multipliers 141 to 14M. The multipliers 141 to 14M multiply the branched symbols individually by codes C1, C2, . . . , CN constituting orthogonal codes and output the resulting signals. As a result, subcarrier signals S1 to SMN for multicarrier transmission by N×M subcarriers are output from the spreader 14. That is, the spreader 14 multiplies the symbols of every parallel sequence by the orthogonal codes, thereby performing spreading in the frequency direction. Codes that differ for every user are assigned as the orthogonal codes used in spreading.
In the case of a downlink (transmission by a base station), a code multiplexer 15 code-multiplexes the subcarrier signals generated as set forth above and the subcarriers of other users generated through a similar method. That is, for every subcarrier, the code multiplexer 15 combines the subcarrier signals of a plurality of users conforming to the subcarriers and outputs the result. A frequency interleaver 16 rearranges the code-multiplexed subcarriers by frequency interleaving, thereby distributing the subcarrier signals along the frequency axis, in order to obtain frequency-diversity gain. An IFFT (Inverse Fast Fourier Transform) unit 17 applies an IFFT to the subcarrier signals that enter in parallel, thereby effecting a conversion to an OFDM signal (a real-part signal and an imaginary-part signal) on the time axis. A guard-interval insertion unit 18 inserts a guard interval into the OFDM signal, an orthogonal modulator 19 applies orthogonal modulation to the OFDM signal into which the guard interval has been inserted, and a radio transmitter 20 up-converts the signal to a radio frequency, applies high-frequency amplification and transmits the resulting signal from an antenna.
The total number of subcarriers is (spreading ratio N)×(number M of parallel sequences). Further, since fading that differs from subcarrier to subcarrier is sustained on the propagation path, a pilot is time-multiplexed onto all subcarriers and it is so arranged that fading compensation can be performed subcarrier by subcarrier on the receiving side. The time-multiplexed pilot is a common pilot that all users employ in channel estimation. In case of uplink, the signals of each of the users are combined on the propagation path and received at a base station.
FIG. 17 is a diagram useful in describing a serial-to-parallel conversion. Here a common pilot P has been time-multiplexed to the front of transmit data. If the common pilot is 4×M symbols and the transmit data is 30×M symbols, then M symbols of the pilot will be output from the serial/parallel converter 13 as parallel data the first four times, and thereafter M symbols of the transmit data will be output from the serial/parallel converter 13 as parallel data 30 times. As a result, the pilot can be time-multiplexed into all subcarriers and transmitted. By using this pilot on the receiving side, fading compensation becomes possible on a per-subcarrier basis.
FIG. 18 is a diagram useful in describing insertion of a guard interval. If an IFFT output signal conforming to M symbols is taken as one unit, then guard-interval insertion signifies copying the tail-end portion of this symbol to the leading-end portion thereof. Inserting a guard interval GI makes it possible to eliminate the effects of inter-symbol interference ascribable to multipath.
FIG. 19 is a diagram showing structure on the receiving side of MC-CDMA. A radio receiver 21 subjects a received multicarrier signal to frequency conversion processing, and an orthogonal demodulator 22 subjects the receive signal to orthogonal demodulation processing. A timing-synchronization/guard-interval removal unit 23 establishes receive-signal timing synchronization, removes the guard interval GI from the receive signal and inputs the result to an FFT (Fast Fourier Transform) unit 24. The FFT unit 24 converts a signal in the time domain to N×M-number of subcarrier signals. A frequency deinterleaver 25 rearranges the subcarrier signals in an order opposite that on the transmitting side and outputs the signals in the order of the subcarrier frequencies.
After deinterleaving is carried out, a fading compensator 26 performs channel estimation on a per-subcarrier basis using the pilot time-multiplexed on the transmitting side and applies fading compensation. In the Figure, a channel estimation unit 26a1 is illustrated only in regard to one subcarrier. However, such a channel estimation unit is provided for every subcarrier. The channel estimation unit 26a1 estimates the influence exp(jφ) of fading on phase using the pilot signal, and a multiplier 26b1 multiplies the subcarrier signal of the transmit symbol by exp(−jφ) to compensate for fading. A despreader 27 has M-number of multipliers 271 to 27M. The multiplier 271 multiplies N-number of subcarriers individually by codes C1, C2, . . . , CN constituting orthogonal codes assigned to users and outputs the results. The other multipliers also execute similar processing. As a result, the fading-compensated signals are despread by spreading codes assigned to each of the users, and signals of desired users are extracted from the code-multiplexed signals by despreading.
Combiners 281 to 28M each add the N-number of results of multiplication that are output from respective ones of the multipliers 271 to 27M, thereby creating parallel data comprising M-number of symbols. A parallel/serial converter 29 converts this parallel data to serial data, and a data demodulator 30 demodulates the transmit data.
FIG. 20 is an explanatory view illustrating the state of MC-CDMA fading fluctuation in a downlink from a base station. A subcarrier signal of a prescribed user that has been spread along the frequency axis is code-multiplexed onto the subcarrier signal of another user and sustains fading that differs for every subcarrier on the propagation path. In the case of an outgoing call from a base station, however, a code-multiplexed subcarrier signal of a different user sustains the same fading FD12 on each subcarrier. As a result, if fading compensation is carried out using channel estimation information estimated for every carrier by means of the pilot symbol, fading sustained by each user can be compensated for simultaneously, as indicated by FD12′, orthogonality of the spreading codes of each of the users can be maintained and user signals will not interfere with one another. Accordingly, using a code having a high degree of orthogonality as the spreading code is extremely effective in an MC-CDMA scheme.
In the uplink, however, each user experiences different degrees of fading F1, F2 as shown in FIG. 21. Consequently, each subcarrier signal experiences independent fading user by user and orthogonality of the spreading code of each user is lost completely. For example, even if fading fluctuation F1 with respect to user 1 is compensated for by a fading compensator in the manner indicated at F1′, fading fluctuation F2 with respect to user 2 becomes as indicated at F2′. Accordingly, in a case where the MC-CDMA scheme for performing spreading in the frequency domain is applied to the uplink, a large deterioration in characteristics occurs.
Further, there are instances where a directional beam is emitted from a base station toward each user to transmit data. In such an instance it is necessary to provide an antenna array and carry out beam forming by causing a beam former to apply an array weight, which differs for every user, to the transmit data. FIG. 22 is a simple structural view of beam forming and illustrates an array antenna 31, a transmit beam former 32 for controlling the directivity of the beam by changing the array weight, and transmitters 331 to 33N for inputting transmit signals to antenna elements ATT1 to ATTN that construct the array antenna. The transmit beam former 32 controls the array weight in accordance with the direction of the user (mobile station), thereby transmitting the beam toward the user upon changing the magnitude and phase of the transmit signal applied to each antenna element.
In a case where a signal to which a weight that differs for every user has been applied is code-multiplexed in order to implement beam forming, each subcarrier will sustain fading that differs for every user, in a manner similar to that the uplink in FIG. 21, if this signal traverses a propagation path that experiences multipath fading. In particular, the larger the angular spread of multipath, the more the fading experienced by each user is different from and independent of that of other users. Even when beam forming is carried out, therefore, the orthogonality of spreading codes is lost and the characteristic deteriorates significantly. If the array weight of each user has been decided in such a manner that the beam will not be directed toward other users in this case, the interference component toward other users is suppressed and therefore the problem of loss of spreading-code orthogonality is eliminated. In actuality, however, the side lobes of the beam interfere with other users and, hence, the influence of deterioration of characteristics due to loss of orthogonality is great.
As shown in FIG. 23, there are also cases where a sector is divided into a plurality, e.g., three, of directional zones A1, A2, A3, and a beam having the same directivity is transmitted by being directed toward those mobile stations MS11 to MS12, MS21 to MS22, MS31 to MS32 present in the same directional zone. In the case of such beam forming, it is necessary to use an array weight that differs for every directional zone. However, if a signal to which a weight that differs for every directional zone is code-multiplexed and the code-multiplexed signal traverses a propagation path that experience multipath fading, then, in a manner similar to that of FIG. 21, each subcarrier will sustain fading that differs for every directional zone. In such case, fading sustained by users in different directional zones becomes independent, orthogonality of spreading codes is lost and characteristics deteriorate markedly. In this case also, if the array weight of each directional zone has been decided in such a manner that the beam will not be directed toward other directional zones, the interference component toward other users is suppressed and therefore the problem of loss of spreading-code orthogonality is eliminated. In actuality, however, side lobes SB of the beam BM interfere with other users, as shown in FIG. 23, and, hence, the influence of deterioration of characteristics due to loss of orthogonality is great.
Furthermore, in a case where each subcarrier has sustained fading that differs for every user, channel estimation must be performed using an individual pilot for each user in order to compensate for fading. With MC-CDMA, channel estimation must be carried out with regard to each subcarrier before despreading is performed. To separate an individual pilot of each user, there is a method of utilizing the pattern of a pilot and a method of shifting the position along the frequency and time axes of the pilot user by user to thereby effect separation. In either case, power allocated to individual channels decreases as the number of individual channels increases. (a) in FIG. 24 illustrates a case where a common pilot is used and (b) of FIG. 24 a case where individual pilots are used. The power of each individual pilot is one-fourth that of the common pilot. The greater the number of individual pilots, therefore, the greater the decline in the accuracy of channel estimation. Further, depending upon the placement and position of the pilot pattern, the channel of each subcarrier must be estimated as by finding channel information between pilots (channel estimation information) by interpolation.